COMMON DESCENT

Listen to Episode 116 on PodBean, YouTube, Spotify, or wherever else you can find it!

Of all the reptiles to evolve marine lifestyles, only one were so strikingly specialized to earn the name “fish reptiles.” This episode, we discuss the long evolutionary history, extraordinary adaptations, diverse lifestyles, and perplexing extinction of Ichthyosaurs.

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The “Fish Reptiles”

The history of evolution has seen many land animals move to the sea, but few secondarily aquatic groups have been as successful, diverse, or famous as the ichthyosaurs. From ~248 million years ago to ~93 million years ago, these reptiles were dominant predators of Mesozoic seas, even as they faced competition from mosasaurs, plesiosaurs, marine crocs, and sharks. They’re known from fossils all over the world, including some exquisite specimens from famous sites like Lyme Regis and Holzmaden, and study of these animals dates back to the beginnings of paleontology, with famous discoveries by Mary Anning and other early paleontologists.

The word “ichthyosaur” means “fish reptile” or “fish lizard,” and it’s well deserved – these were very fish-like animals, exceptionally well-adapted for aquatic life. They had hydrodynamic bodies, powerful tails for swimming, four flippers for steering, and fossilized impressions of soft tissue reveal that many species had tall tail flukes and even dorsal fins. But not all of them had the classic fish-shaped body: early ichthyosaurs were often long and slender, likely using anguilliform (eel-like) swimming by undulating their whole bodies, while later forms looked and moved more like fast-swimming fish and sharks.

Perhaps the most extreme ichthyosaur adaptations are seen in their limbs. Like most secondarily aquatic vertebrates, ichthyosaurs’ flippers are specially adapted arms and legs. But whereas the skeletal structure in the flippers of a whale, seal, or sea turtle (for example) still look roughly like hands and feet, ichthyosaur flippers are often bizarre. They commonly exhibit hyperdactyly (extra fingers) and hyperphalangy (super-long rows of finger bones), and the bones in their flippers often converge on the same round-ish shape, resulting in flipper skeletons that look like cobblestone paths.

Origins and Extinctions

Exactly where ichthyosaurs fit on the reptile family tree is unclear – most recent studies identify them as an early branch of diapsid reptiles – but they aren’t alone. Closely related to them are the ichthyosaur-like Hupesuchians and recently-discovered Narorostrans from China. True ichthyosaurs got started very early in the Triassic Period, shortly after the end-Permian mass extinction, and they achieved an incredible diversity in the Middle and Late Triassic. During this time, they evolved a wide range of lifestyles, body shapes, and sizes, from tiny one-meter animals to some of the largest animals of all time, with some groups (like Shonisaurus) reaching 20m long or more.

Ichthyosaurs lost a lot of their diversity in the end-Triassic extinction, but they continued to be major predators in the Jurassic. Indeed, many of the most famous and well-known ichthyosaurs are from the Jurassic, including Ichthyosaurus, Stenopterygius, and Ophthalmosaurus.

Our understanding of Cretaceous ichthyosaurs has changed dramatically in recent years. Formerly, they were thought to have barely made it past the end-Jurassic, with only stragglers making it into the Cretaceous before fizzling out in the Cenomanian (~93 million years ago). But recent discoveries have shown that ichthyosaurs were unexpectedly diverse until shortly before their extinction, which was likely linked to global changes to oceans and ocean life throughout the Cenomanian.

The Lives of Ichthyosaurs

Ichthyosaurs were predators. Many seem to have hunted soft-bodied prey like squid and small fish, based on their slender pointy teeth and the abundance of those animals’ remains found in ichthyosaur gut contents. But many species had round teeth great for crushing hard-shelled prey, and still others had large slicing teeth for taking down big prey the same way sharks or orcas still do today. Some ichthyosaurs, like the large-eyed Ophthalmosaurus, might have dove to the depths for their food like sperm whales do today. Some ichthyosaurs have also been suspected of using suction-feeding, though recent evidence has called that into question.

Exceptionally-preserved specimens have given us incredible insights into ichthyosaur biology. Soft tissue impressions not only reveal the shapes of their bodies, but recent research has even identified hints of coloration and blubber! And, famously, there are numerous specimens of pregnant ichthyosaurs, fossilized with fossil embryos inside them. Ichthyosaurs are among the oldest reptiles known to have given live birth.

Learn more

In addition to the links in the article above, here are some more places to learn about ichthyosaurs:

Overviews:
Motani, 2005. Evolution of Fish-shaped reptiles (technical, paywalled)
Sander, 2000. Ichthyosauria: their diversity, distribution, and phylogeny (technical, paywalled)

Cretaceous ichthyosaurs:
The Cretaceous ichthyosaur revolution, Tetrapod Zoology, Part 1 and Part 2 (semi-technical)
Zammit, 2012. Cretaceous Ichthyosaurs: Dwindling Diversity, or the Empire Strikes Back? (technical, open access)

Fischer et al 2016. Extinction of fish-shaped marine reptiles (technical, open access)

Motani et al 2014. Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils (technical, open access)

Lomax et al 2018. A giant Late Triassic ichthyosaur from the UK (technical, open access)

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 51 – Mosasaurs
• Episode 72 – Plesiosaurs
• Episode 70 – Convergent Evolution
• Episode 71 – The Western Interior Seaway

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